1. Field
Optical communication devices and systems, particularly optical wavelength converters, are used for metro and long-haul communications.
2. Prior Art
The field of optical communication extensively uses Dense Wavelength Division Multiplexing (DWDM) in which a plurality of information channels corresponding to plurality of different wavelengths are inserted, in parallel, into the same optical fiber. Communication systems have a need to transmit information from one channel having a certain wavelength to another channel with another wavelength. In such a situation (channels operate in different wavelengths) there is a need for wavelength converters to allow the transmission of the information from one information channel to another information channel and without the need for Optical-Electrical-Optical (O-E-O) converters.
Wavelength converters may be used also for switching purposes when the wavelength change results with a different port from which the radiation is emitted by wavelength-sensitive demultiplexers (WDM or DWDM).
FIG. 1 is a schematic illustration of a prior-art Mach Zhender Interferometer (MZI) wavelength converter 400. This converter is used to convert signals carried by one wavelength to be carried by another wavelength. Wavelength converters are implemented in the metro area and in the long-haul networks as well. The wavelength converter is designed to convert information generating pulses 402 of wavelength λ1, at terminal 404, into converted information pulses 406 of wavelength λ2, at terminal 408. Continuous Wave (cw) radiation 410, having a wavelength λ2, is inserted at terminal 412 and is split by coupler 414 into cw radiation propagating in branches of radiation guides 416 and 418. The radiation in branched 416 and 418 passes through Solid-state Optical Amplifiers (SOA) 422 and 420, respectively, serving as Non Linear Elements (NLE). SOAs 420 and 422 are adjusted to produce relative phase shifts between the cw radiation in guides 416 and 418 for causing the radiation from guides 416 and 418 to be combined destructively in coupler 424. Accordingly, when no signal 402 is present in terminal 404, there is no output signal 406 at port 408.
When signal 402 having a wavelength λ1 is received by terminal 404, it is coupled by coupler 426 into guide 418 and passes through SOA 420. For the time duration in which signal 404 passes through SOA 420, it causes a phase change of π radians to the cw radiation propagating in the opposite direction in SOA 420. In this case the cw radiation from guides 416 and 418 is combined constructively, by coupler 424, to produce pulse 406 at output 408 having a wavelength λ2 and a time duration equal to that of pulse 402. Accordingly, converter 400 converts pulses 402 of wavelength λ1 at port 404 into similar pulses 406, of wavelength λ2, at port 408. The components of the converter of FIG. 1 are well known.
The wavelength converter of FIG. 1 may have one or more of the following disadvantages:
1. The device is phase sensitive and thus electric current injected into SOAs 420 and 422 should be controlled, separately, to each of them to maintain the desired phase relations and to compensate for phase changes resulted from environments changes and/or various drifts in the values of some parameters of device 400, such as gain drifts of SOAs 420 and 422.
2. In the absence of signals 402 at port 404 and in order to produce zero output signals 406 at port 408, there is a need to maintain independent gain and phase relations between amplifiers 422 and 420 at branches 416 and 418, respectively. However the gain and the phase shifts of amplifiers 422 and 420 are dependent parameters, resulting in a situation that may be difficult to control.
3. The design of the device requires two SOAs which may reduce manufacturing yield and may increase manufacturing cost and complexity.
U.S. patent application Ser. No. 10/845,149 entitled “All optical phase insensitive wavelength converters, apparatus systems and methods” filed May 14, 2004” by Arie Shahar et al. and published Nov. 25, 2004 as Publication Nr. 20040233513 shows a wavelength converter which has one or more of the following drawbacks:
1. It is sensitive to the polarization orientation of the converted signal.
2. It includes an attenuator in its optical loop that attenuates the converted signal.
International patent application PCT/US2004/028588 (International Publication Number WO 2005/022706 A2) by Paul R. Prucnal, filed Sep. 2, 2004 (priority date of Sep. 2, 2003—U.S. Ser. No. 60/499,536) discloses a wavelength converter that suffers from the following disadvantages:                1. The device is polarization sensitive.        2. The device suffers from high loss due to a coupler in its optical loop. This coupler should have a high coupling coefficient to efficiently couple the converted signal into the optical loop in order to have enough power to produce polarization rotation in the Non Linear Element (NLE) of the optical loop. This means that that the loss in the optical loop is at least equal to the high coupling ratio of the above mentioned coupler.        3. The device suffers from high pattern distortion due to the high power of the converted signal injected from one side only into the NLE in the optical loop.        4. The device suffers from high gain instability due to the high power converted signal that causes the gain of the NLE to be pattern dependent.        